A portable Internet system based on Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard uses a wide frequency bandwidth and thus transmits much data for a short time. Moreover, all users can efficiently use a channel by sharing the channel.
Data transmission/reception between a Mobile Station (MS) and a Radio Access Station (RAS) is performed by the Orthogonal Frequency Division Multiplexing (OFDM) scheme or the Orthogonal Frequency Division Multiple Access (OFDMA) scheme, and the mobility of the mobile station is supported by a multi-cell structure.
FIG. 1 is a diagram schematically illustrating a configuration of a portable Internet system.
Referring to FIG. 1, when a mobile station 110 moves from a cell area 130 of a serving RAS 120, to which service is being provided currently, to cell areas 131 and 132 of other RASs 121 and 122, a portable Internet system prevents service provided to the mobile station 110 from being stopped by performing handover.
In the portable Internet system, a mobile station performs cell reselection, handover decision and initiation, synchronization with a target RAS, and ranging, upon handover. Upon handover, a mobile station is required to transmit/receive information regarding handover to/from a serving RAS and a target RAS, and thus, delay occurs. Herein, transmission of fast ranging information message (hereinafter referred to as fast ranging IE) corresponds to a fast ranging operation for minimizing delay due to ranging, among operations associated with handover.
When a handover procedure defined in IEEE 802.16e standard is normally performed, fast ranging is capable of being performed. However, a mobile station decides handover, and thereafter arbitrarily decides handover initiation, or handover initiation can be delayed due to another cause. In this case, handover is performed between a mobile station and a target RAS after an action time, and thus, the mobile station cannot receive fast ranging IE transmitted by the target RAS. At this point, a fast ranging operation is not performed, causing the delay of handover.
Also, when a signal state of a serving RAS is rapidly changed or very bad during a handover decision and initiation operation, a mobile station cannot exchange a message with the serving RAS. In this case, a message transmitted/received between the mobile station and the serving RAS is omitted, causing the delay of handover.
Moreover, when the signal state of the serving RAS is well changed and thus handover is not required, handover is cancelled during the handover decision operation, and the mobile station can continuously communicate with the serving RAS. However, since an action time for the transmission of fast ranging IE is decided during the handover decision operation, the target RAS allocates fast ranging IE to the action time irrespective of a state between the mobile station and the serving RAS, and thus, a wireless resource can be wasted.